The information, including box 6 "Voluntary Withholding," on the form 1099-SSA you received for social security benefits paid is entered in the "Wages & Income" section.
- Scroll down to "Retirement Plans and Social Security"
- Open the "dropdown" menu
- Start/Revisit "Social Security..."
- There you will find the boxes matching your Form 1099-SSA, including Box 6